Connecting a new shower to old tub drain

I've removed the old tub and want to build a shower pan on the slab foundation. I want to connect the new shower drain to the existing tub drain. The old drain is 1.5 inches and the p-trap is below ground level (there's a square cut out of the slab at the drain location). My question is since old drain is 1.5 inches and showers use 2.0 inch pipe, how can I do this without excavating the earth all along the pipe to reach a 2.0 line? I know reducing 2.0 to 1.5 is not a good idea but I'm at a loss now for ideas. thanks!

You answered your own question, crack some concrete. You normally want the shower drain centered on the shower, too. Unless you are rearranging the location of the new shower relative to the existing tub drain, you need that moved as well. If you have to get the place inspected and permitted (and if you have the house inspected as part of an eventual sale), you'll need the 2" line anyways. My unprofessional experience.